I'm considering using node.js + expresso + couchdb. What worries me is the CPU overhead on the servers for using javascript. How much CPU is needed to run such

I'm considering using node.js + expresso + couchdb. What worries me is the CPU overhead on the servers for using javascript. How much CPU is needed to run such an app or a service like Trello which I heard is (was?) using node.js. The scalability is a Maserati problem. It would just like to know how steep is the curve of the required power relative to the number of users for a small messaging app with one database per user (a.k.a. zimbra) ?